Current ultra-high frequency early-warning radar (UEWR) systems including phased array antennas are deployed in several locations around the world and have been operated by the United States military for decades. A phased array antenna includes numerous radio frequency (RF) radiating elements each connected in an assembly of solid state electronics which permits transmission and reception of RF signals. The assembly is termed a solid state module (SSM). For UEWR, each of 32 RF SSMs are fed by sub-array modules. Beams can be formed by shifting the phase of signals emitted from each radiating element to provide constructive and destructive interference. Each antenna element is delayed by the correct amount so that a wave front arriving from a given direction is aligned to receive the signals or the energy is directed to transmit signals. Each SSM includes a set of selectable for phase adjustments for transmit and receive beam steering in and amplitude for receive array pattern shaping. Performance of a phased array radar system is strongly dependent on the calibration of signal transmission lines in and between the RF modules and sub-array modules.
Calibration of phased array radar systems generally involves RF alignment at the sub-array level and the element level. In current UEWR systems, sub-array calibration involves alignment of transmit paths and receive paths for up to 80 sub-arrays. Elements of current UEWR systems are not calibrated in-situ. Rather, element calibration involves factory alignment of the modules.
Sub-arrays of UEWR systems have heretofore been calibrated manually by technicians who make manual adjustments to the sub-array line lengths based on RF measurements. Sub-array level alignment is performed by physically adjusting the line lengths using built in trimmers based on external measurement equipment, such as a special purpose radio frequency monitor (RFM) subsystem, for example.
Referring to FIG. 1, a prior art UEWR system 100 includes up to 80 sub-arrays. Each sub-array includes a sub-array driver assembly (SDA) 102. Each SDA 102 distributes RF signals from a single line 123 into four separate lines 104 to transmit signals and combines received signals from the four separate lines 104 onto the single line 123. The transmit signal is separated from the receive signal with the circulator 121. Each of four lines 104 is connected to a 1:8 splitter/combiner 106 which supplies/receives RF signals from eight solid state modules (SSM) 108. A total of 32 SSMs 108 are serviced with one SDA 102.
According to an aspect of the present disclosure, each SDA 102 also includes a receive path line 118. The receive path lines 116 are coupled to receive beam former (RBF) circuitry 107 and receiver circuitry 109
To align the SDA 102 for receiving, the traditional method involves configuring an RF Monitor Injection (RFM) 116 sub-system to inject a known signal to the SDA 102. Each of the sub-array lines to a common receiver is measured, and their lengths are adjusted manually to achieve alignment. Similarly, transmit lines which supply RF signals to each SDA 102 are aligned by measuring the phase of the RF signal using the RFM 116 in a receive capacity. With the current calibration system, measurements are performed manually with external equipment, and adjustments are made physically using a bank of line stretchers.
Using the current method, the front end of the radar, which is beyond the RFM 116, cannot be calibrated. Rather, alignment of individual antenna elements, solid state modules 108, and cabling is performed by controlling manufacturing tolerances.
Thus, the traditional method of calibrating radar involves costly time consuming manual operations, and is limited in its ability since it cannot correct for all errors in the RF path to the element. The limited calibration accuracy limits sensitivity and tracking performance of radar systems. Compensation with additional SSMs 102 further adds costs associated with using the traditional method.
Future radar technology upgrades may include digital beamforming and other improvements. Such technology upgrades may require periodic real time calibrations, which are not possible using the present manual RFM calibration methods.